Clinical trial data has established the prophylactic efficacy and safety of pneumatic compression devices among medical and surgical patients. Eleven different types of compression devices are approved by the FDA but no evidence exists to allow comparison of one to another and there is no clinical basis for selection of a particular device. Three criteria have been offered by manufacturers of these devices to claim superiority: mean blood flow velocity, peak blood flow velocity and flow volume. We will measure these parameters in 15 healthy volunteers then develop a relative ranking scale for each of the three factors believed to be associated with device effectiveness. A fourth criteria, increased fibrinolytic effect, is being evaluated at two other institutions and we will not replicate this work. A clinical study is being conducted concurrently to determine the prophylactic effectiveness of these same devices in patients. Combining the data from these studies will allow us to determine which of the three evaluation factors are associated with clinical effectiveness.